1. Field of the Invention
This invention is directed to methods for extending the cycle life of solid, secondary electrolytic cells during recharge of the cells.
2. State of the Art
Electrolytic cells comprising an anode, a cathode and a solid, solvent-containing electrolyte are known in the art and are usually referred to as "solid electrolytic cells". One class of solid electrolytic cells are rechargeable (secondary) lithium cells which comprise a solid electrolyte interposed between an anode comprising lithium and a cathode which comprises materials suitable for recycling (recharging) the cell after discharge.
A solid, secondary battery typically comprises several solid, secondary electrolytic cells wherein the current from each of the cells is accumulated by a conventional current collector so that the total current generated by the battery is roughly the sum of the current generated from each of the individual electrolytic cells employed in the battery. Such an arrangement enhances the overall current produced by the solid, secondary battery to levels which render such batteries commercially viable.
However, one problem encountered with the use of solid, secondary electrolytic cells in such batteries is limited cycle life for the battery, i.e., the number of rechargings the battery can accept before the battery is no longer able to maintain acceptable levels of capacity. Specifically, the cycle life of the solid, secondary battery is related to the cycle lives of the individual electrolytic cells comprising the battery. In general, when one of the electrolytic cells in the battery ceases to maintain acceptable levels of capacity, the battery must drain more current from the remaining electrolytic cells so as to produce the same overall level of current from the battery which results in a reduction of the capacity of the remaining electrolytic cells in the battery. In turn, this results in a significant reduction in the cycle life of these cells and hence that of the battery.
Without being limited to any theory, it is believed that reduced cycle life in secondary lithium electrolytic cells containing a solid, solvent-containing electrolyte interposed between a lithium anode and a cathode arises, in part, from lithium dendrite growth on the cathodic surface during recharging of the discharged cell. Specifically, it is believed that during each recharging cycle, lithium dendrites form on the surface of the cathode and expand or grow outward during recharging. Because the solid, solvent-containing electrolyte interposed between the lithium anode and the cathode is pliable, dendrite growth is typically outward as it pushes aside the pliable electrolyte. Additionally, because dendrite growth is cumulative over repeated charging cycles, these growing dendrites will eventually contact the anode resulting in microshorts in the electrolytic cells. This accumulation of microshorts eventually shorts the electrolytic cell thereby leading to termination of its cycle life because the cell is unable to accept charge.
In view of the above, the art has been searching for methods which would reduce or eliminate lithium dendrite growth during recharging of the electrolytic cell so as to prolong the cycle life of the cells and hence that of the battery.